New Walker Advice

Bearings

To take a bearing, you will need a map and compass.

The grid lines on your map point to grid north whilst your compass needle will point
to magnetic north. The offset between these will vary depending on where you are
in the world. It will also vary with time. The offset will be printed on the key of your map.
Near Oxford, the offset of magnetic north is about 3° west of grid north.

If you want to use bearings over any distance then you will need to make allowance for this
difference. Near Oxford, not adjusting will give an error of about 100 yards over a mile.
The instructions below explain how to make the adjustment but that step can be skipped.

True north is the direction of the north pole. The grid on OS maps is based on true north at the
west edge of the SP grid square (about 50km west of Oxford). Because of the curvature of the earth
grid north has a greater offset from true north the further east or west you travel from this location.

Walking in a particular direction

If you know your current location on your map, you can take a bearing and use it to walk to
an object on your map. If your target is any distance then you should regularly check that you
are going in the right direstion.

Place your compass on the map so that the edge passes through your current location and the
point which you want to walk to. The arrow on the base plate of the compass should point in
the direction you want to travel.

Turn the compass dial so that the arrow on the dial points to north on the map (the lines on
the dial will also be parallel to the north-south grid lines on the map). You can remove the
compass from the map now.

Adjusting for magnetic north offset (optional): If the offset of magnetic north from grid north is west
then you need to turn the dial anti-clockwise by the relevant number of degrees (3 near Oxford).
If the offset is east then you would need to turn the dial clockwise.

Turn the compass so that the north pointing needle (normally coloured red) lines up with the
arrow on the compass dial. The arrow on the compass base plate will then point in the direction
you should walk.

Using bearings to find your location (triangulation)

If you do not know where you are but can see several landmarks, such as a church spire, a pylon
or the corner of a wood, you can take bearings on them to work out your location. Repeat the following
procedure for at least two landmarks. The more landmarks you are able to use the more certain you
can be of your location.

Line up the arrow on the compass base plate with the landmark. Turn the dial so that the north
pointing needle (normally coloured red) lines up with the arrow on the dial.

Adjusting for magnetic north offset (optional): If the offset of magnetic north from grid north is west
then you need to turn the dial clockwise by the relevant number of degrees (3 near Oxford).
If the offset is east then you would need to turn the dial anti-clockwise.

Locate the landmark on your map and place the edge of the compass base plate on it. Keeping the
compass fixed on this point, rotate the compass (not the dial) so that the arrow (and the lines)
on the dial line up with the north-south grid lines on the map.

Draw a line along the edge of the compass - you are somewhere along this line.

Once you have drawn two lines, they should cross - this is your location. If you take bearings on
more than two points, all the bearings should cross at roughly the same point.